It is known to oxidatively dehydrogenate alkanes, such as ethane, to produce ethylene in an oxidative dehydrogenation (oxydehydrogenation; ODH) process. Examples of ethane ODH processes, including catalysts and other process conditions, are disclosed in U.S. Pat. No. 7,091,377, WO2003064035, US20040147393, WO2010096909 and US20100256432. Mixed metal oxide catalysts containing molybdenum (Mo), vanadium (V), niobium (Nb) and optionally tellurium (Te) can be used as such oxydehydrogenation catalysts. The dehydrogenated equivalent of ethane may be further oxidized under the same conditions into acetic acid.
Presently, the process can achieve over 90% ethylene selectivity to ethylene at over 50% conversion. The process is also typically producing acetic acid (AA, ˜5 mol % selectivity) together with carbon dioxide (CO2).
In the above processes, the acetic acid thus produced is generally considered as waste products. Although they could be condensed together with water from the reactor effluent as an aqueous carboxylic acid (ca. 10 wt %) stream, the low relative volatility of carboxylic acids to water renders distillative separation of carboxylic acid and water troublesome, as this would require very large condensate recycle and/or separation trains.
Accordingly, current ODH processes are designed for minimal acetic acid co-production. However, proper valorization of acetic acid could allow “relaxing” acetic acid specifications, thereby widening the operating window to accommodate e.g. higher pressures, lower temperatures and/or easier management of heat release and explosion risks in the ODH process. For example, relaxing the acetic acid specification to 10 mol % selectivity could deliver an aqueous side-stream containing 20 wt % acetic acid.
WO1998005620 describes an integrated process for the production of acetic acid and/or vinyl acetate which comprises the steps: (a) contacting in a first reaction zone a gaseous feedstock comprising ethylene and/or ethane and optionally steam with a molecular oxygen-containing gas in the presence of a catalyst active for the oxidation of ethylene to acetic acid and/or ethane to acetic acid and ethylene to produce a first product stream comprising acetic acid, water and ethylene (either as unconverted ethylene and/or as co-produced ethylene) and optionally also ethane, carbon monoxide, carbon dioxide and/or nitrogen; (b) contacting in a second reaction zone in the presence or absence of additional ethylene and/or acetic acid at least a portion of the first gaseous product stream comprising at least acetic acid and ethylene and optionally also one or more of water, ethane, carbon monoxide, carbon dioxide and/or nitrogen with a molecular oxygen-containing gas in the presence of a catalyst active for the production of vinyl acetate to produce a second product stream comprising vinyl acetate, water, acetic acid and optionally ethylene; (c) separating the product stream from step (b) by distillation into an overhead azeotrope fraction comprising vinyl acetate and water and a base fraction comprising acetic acid; (d) either (i) recovering acetic acid from the base fraction separated in step (c) and optionally recycling the azeotrope fraction separated in step (c) after partial or complete separation of the water therefrom to step (c), or (ii) recovering vinyl acetate from the azeotrope fraction separated in step (c) and optionally recycling the base fraction separated in step (c) to step (b), or (iii) recovering acetic acid from the base fraction separated in step (c) and recovering vinyl acetate from the overhead azeotrope fraction recovered in step (c).
WO2000069802 describes a process for the production of vinyl acetate monomer comprising the steps of: (1) contacting a gaseous feed mixture of ethane or ethylene or ethane/ethylene, steam and a molecular oxygen containing gas in the presence of a first catalyst active for oxidation of ethane, ethylene or ethane/ethylene to produce a selective stream of acetic acid, ethylene, carbon dioxide and water and (2) converting a second feed mixture comprising ethylene, acetic acid and oxygen to vinyl acetate monomer in the presence of a second catalyst active for the production of vinyl acetate, wherein said process does not include an intermediate separation step to remove the CO between the two reaction steps.
WO2001090042 describes an integrated process for the production of vinyl acetate which comprises the steps of: a) contacting in a first reaction zone a gaseous feedstock comprising essentially ethane with a molecular oxygen-containing gas in the presence of a catalyst to produce a first product stream comprising acetic acid and ethylene; b) contacting in a second reaction zone the first gaseous product stream with a molecular oxygen-containing gas in the presence of a catalyst to produce a second product stream comprising vinyl acetate; c) separating the product stream from step (b) and recovering vinyl acetate from the product stream from step (b).
These prior art processes generally suffer from poor selectivity to vinyl acetate due to the presence of considerable amounts of water in the effluent of the oxidative dehydrogenation reaction.
It is desirable to provide an ethane oxidative dehydrogenation process, wherein the acetic acid side stream thus produced is valorized by conversion to vinyl acetate in a technically advantageous, efficient and economically affordable manner.